Oilfield operations may be performed in order to extract fluids from the earth. When a well site is completed, pressure control equipment may be placed near the surface of the earth including in a subsea environment. The pressure control equipment may control the pressure in the wellbore while drilling, completing and producing the wellbore. The pressure control equipment may include blowout preventers (BOP), rotating control devices, and the like.
The rotating control device or RCD is a drill-through device with a rotating seal that contacts and seals against the drill string (drill pipe, casing, drill collars, kelly, etc.) for the purposes of controlling the pressure or fluid flow to the surface. The RCD may have multiple seal assemblies and, as part of a seal assembly, may have two or more seal elements in the form of stripper rubbers for engaging the drill string and controlling pressure up and/or downstream from the stripper rubbers. For reference to an existing description of a rotating control device incorporating a pair of opposed sealing elements, please see U.S. Pat. No. 6,230,824 entitled “Rotating Subsea Diverter”, granted May 15, 2001, the disclosure of which is hereby incorporated by reference.
The seal elements in the RCD or other pressure control equipment have a tendency to wear out quickly. For example, tool joints passing through the sealing element may cause failure in the sealing element via stresses eventually causing fatigue and/or via chunks of seal material tearing out of the sealing element. In high pressure, and/or high temperature wells the need is greater for a more robust and efficient seal element.
In subsea RCDs, the RCD may have two or more seal elements which may be stripper rubbers. One seal element may be at an inlet to the RCD and exposed to a riser above the RCD. A second seal element may be located downstream of the first seal element and may be exposed to the wellbore pressure below. This second seal element may seal the wellbore pressure in the wellbore.
As the drill string is run into, and/or out of the RCD, this movement may have certain effects that could enhance the risk of failure to a sealing element. The axial movement whether upward or downward will cause the drill string to move through chambers or regions between two sealing elements. At a first interval in which a tool joint of larger volume than the drill string enters the chamber, the chamber will vacate some fluid through a sealing element in order to account for the increased volume of the tool joint. At a second interval when such tool joint passes out of the chamber there is less fluid in the chamber (and less volume of tool in the chamber) thereby causing a reduction in pressure or suction within the chamber. Optionally, at a third interval, a still larger volume tool joint may enter the chamber causing further vacation of fluid. Optionally, at a fourth interval, as the relatively larger volume tool joint emerges from the chamber a further reduction in pressure may result within the chamber. Accordingly, it is possible that suction or vacuum pressure may build up in the chamber between the first sealing element and the second sealing element. This vacuum pressure may enhance the risk of failure to the sealing element(s). There is a need for an improved RCD for controlling the pressure differential between the sealing elements in a subsea RCD.